Farm tractors have been used for over a hundred years and have been a major factor in improving the efficiency of agriculture. Further, farm attachments have evolved around the three point hitch to provide a high level of efficiency and flexibility. Recently, concerns about the cost, safety, maintenance and negative environmental impact of fossil fuel burning tractors has kindled an interest in electric tractors.
U.S. Pat. No. 4,347,907 to Downing, Jr. (1987) is an electric tractor driven by A.C. motors controlled by cycloconverters. The invention mentions a technique for varying the wheel speeds as a function of steering and suggests powering farm implements with more A.C. motors. The invention also mentions an A.C. outlet for non-motor driven accessories only and does not incorporate the use of hydraulic motors for torque at the wheels.
U.S. Pat. No. 4,662,472 to Christianson et al (1987) proposes a nominal DC voltage of 128 volts to drive the tractor wheels via a single motor mounted on the frame. A second electric motor is mounted to a front frame member. This electric motor is used to drive a power-take-off shaft and hydraulic articulation steering system. Articulated steering is not ideal for operating rear mounted farm implements because turning applies undue sideways forces on the parts making ground contact.
U.S. Pat. No. 5,743,347 to Gingerich et al (1998) discloses a lawn and garden tractor having independent electric motors for each driven wheel. Each of the rear wheels is provided with a respective speed sensor. A control system in the tractor feeds extra power to one of the two motors when a wheel encounters an obstacle. Also mentioned is a method of providing a hydraulic lift for attachments but no power take off (PTO) provision is mentioned therefore all attachments must supply their own power. One embodiment of this design incorporates swivel wheels in the front which are not practical for row crop farming. The suggested electronic control system is highly sophisticated with many expensive components that could fail or need modifying to provide for the wide range of conditions in which farm tractors must operate. Further, Gingerich suggests measuring resistance in the field windings of each drive motor to gauge the speed at each wheel, however, he does not take into account the resistance to motion at the wheel that could affect accuracy. Further the patent mentions nothing about how it would support the use of standard farm implements except for a self propelled weeder.
U.S. Pat. No. 5,890,555 to Miller (1998) describes an electric vehicle and more specifically a regenerating system for braking but does not include any solutions for slow speed agricultural use.
U.S. Pat. No. 6,089,341 to Gingerich et al (2000) appears to be a refinement of U.S. Pat. No. 5,743,347 by Gingrich (1998) that still does not propose the use of hydraulics to apply torque to the drive wheels or how to attach standard farm implements.
U.S. Pat. No. 6,454,032 to Teal, et al. (2002) describes a method of balancing and controlling the propulsion and steering of a lawn mower but does not propose how the machine would propel or connect with modern farm implements.
U.S. Pat. No. 7,828,099 to Heckeroth (2010) discusses a generic approach to putting batteries and an electric motor on rails but has no suggestions on how to make it work or how to integrate it with modern farming technologies. The invention also does not suggest the integration of hydraulic power in the configuration that would enhance its performance.
U.S. Pat. No. 7,040,445 to Ishii (2010) suggests a method of controlling the steering of a hydraulic driven machine. It does not propose any solutions for making the machine function in farming or gardening applications.